1. Field of the Invention
The present invention relates to hoist control systems generally and more particularly to a hoist control system having limit switches to maintain hoist travel within predetermined limits.
2. Description of the Prior Art
Hoists are known which limit their chain or rope travel to a predetermined range by the use of various limiting devices. Such devices include trip switches for shutting off hoist power. These switches are actuated by trip members located on the chain or rope of the hoist. As the hoist travels to a predetermined raising or lowering position the trip member at the appropriate end of the chain or rope will actuate the trip switch mounted proximately to the chain or rope guide member. The trip switch will then shut off power to the hoist motor to thereby stop the hoist.
Large capacity pneumatic hoists utilize plunger actuated, normally open, valve members mounted in the vicinity of the chain guide as trip switches and enlarged beads on opposite ends of the chain or rope as the trip members. When the enlarged bead on the chain or rope depresses the valve plunger, the normally open valve closes. Since this valve is connected in line with the hoist motor, power to the motor is effectively blocked thereby and the hoist is stopped.
Small capacity pneumatic hoists usually do not provide limit switches. Instead they rely on a brake clutch arrangement whereby the clutch slips upon a predetermined load being applied to the hoist. Thus whenever either end of the chain or wire rope interfere with the chain guide the clutch slips and further hoist movement is prevented. However, the motor is not shut off and provisions have to be made for dissipating any heat generated in the brake clutch.
The above described systems for limiting the range of hoist travel are beset with specific problem areas. Since the trip members are located on the chain or wire rope of the hoist, these members are always exposed to the work area external of the hoist where the trip members can be moved or damaged by the load being lifted or by other objects located in the work area. The trip switches also can be damaged by impact with the lifted load. Where a brake clutch arrangement is used, some added structure is required to dissipate the heat generated in the brake clutch. This added structure adds to the cost and longevity of the hoist.
These problems and others are solved by the present invention which provides an inexpensive system of limiting hoist travel to predetermined limits which system is safely enclosed within the hoist assembly and which provides an accurate and easily adjustable range of hoist travel.